Crosslinked polymers such as crosslinked polyethylene (PE) and more particularly ultrahigh molecular weight polyethylene (UHMWPE) have found extensive use in implant devices such hip, knee and shoulder implants. Such implants typically include a ceramic or metallic piece which articulates against a UHMWPE article or bearing surface. UHMWPE has been the polymer of choice for its high strength, biocompatibility, and wear resistance. Crosslinking the UHMWPE has been found to improve wear resistance but typically at the cost of diminished oxidative resistance particularly when no post processing of the crosslinked polymer such as heat treatment including annealing and remelting is crosslinking is typically accomplished by exposing the UHMWPE to gamma, beta or x-ray irradiation which causes scissions of the C—C and C—H bonds and creates free radical polymer molecules. These free radicals can undergo recombination and rearrangement reactions to form crosslinks. However, some free radicals may remain in the polymer material following irradiation, which could potentially combine with oxygen, nitric oxide, hydrogen peroxide, hypochlorite anion, and nitrite whether present in the outer environment or in the body to cause oxidation of the polymer material.
Several techniques have been developed in order to reduce the amount or number of free radicals created by crosslinking. One method of reducing the free radicals due to crosslinking has been to expose the crosslinked polymer heat treatments such as annealing or melt techniques that usually raise the temperature of the crosslinked polymer composition to slightly below, at or slightly above the melting point of the polymer. This improves the mobility of the free radicals within the polymer and allows some of the free radicals to form additional crosslinks or other bonds thereby quenching the free radicals. Even with such heating methods, some free radicals persist in the polymer. Another approach has been to incorporate an antioxidant into the polymer whether by blending the antioxidant with unconsolidated UHMWPE raw material or through infusing or doping after the UHMWPE raw material has been consolidated. Using both an antioxidant and annealing or melt technique is yet another approach to improving the oxidation resistance of the polymer, as disclosed for example in U.S. Patent Publication Nos. 20070059334 published Mar. 15, 2007 and 20100029858 published Feb. 4, 2010.
Melt techniques and antioxidant incorporation does impose a cost however. Melt techniques can result in diminishment of the crystalline structure of the polymer which can result in some loss of desirable mechanical properties. Incorporation of antioxidants can result in reduced amount of crosslinking per unit of radiation as the antioxidant quenches the free radicals before they can form crosslinks. To compensate for the reduction of crosslinking due to antioxidant scavenging of the free radical, higher radiation doses can be used. However, using higher doses can detrimentally impact the structural integrity of the polymer by causing excessive bond scissions. This is one reason why some choose to consolidate and irradiate the polymer prior to doping with an antioxidant.
The preferred antioxidant for use with implantable polymeric articles is vitamin E or alpha tocopherol due to its biocompatibility and acceptable scavenging of free radicals. Other antioxidants may also be suitable for use with implantable polymeric articles whether alone or in combination which allow the polymer to achieve higher crosslink density per unit of radiation and/or provide acceptable and even improved oxidation resistance.